Gas chromatography is the separation of a mixture of chemical compounds due to their migration rates through a chromatograph column. This separates the compounds based on differences in boiling point, polarity, or molecular size. The separated compounds then flow across a suitable detector such as a thermal conductivity detector (TCD) that determines the concentration of each compound represented in the overall sample. Knowing the concentration of the individual compounds makes it possible to calculate certain physical properties such as BTU or specific gravity using industry-standard equations.
Modern gas chromatographs use multiple valves and columns to split the separation of components into several sub-processes in order to significantly speed up the analysis times and to improve the separation and isolation of individual components. The timing of the valve switching in a gas chromatograph is very important in order to ensure that the change in the analytical flow path occurs after all of one component has left a column, but before any of the next component has left the column. For example, with reference to FIG. 1, the valve timing should occur after component n-C5 has left the column, but before the next component, C6, begins to elute.
Over an extended period of operation (generally several months to years), contamination in the flow path and changes to the performance of the column can cause the time required for a component to exit the column (elution time) to change. Since the valve timing in a gas chromatograph is typically a fixed value, slowly one of the components will be gradually “cut” by the valve switching. When a component is cut by the valve switching, some of the cut component does not reach the detector when the gas chromatograph is configured to measure it and thus the measurement is incorrect. For example, FIGS. 2A and 2B illustrate a pair of examples where incorrect valve timing can generate errors. In the first example, FIG. 2A shows the valve timing occurring too late. In this example, some portions of C6 will be included in the detection of the previous peak (n-C5). In the second example, FIG. 2B shows valve timing occurring too early. In the second example, some portions of n-C5 are not fully eluted, and thus will not be detected by the detector of the gas chromatograph.